Intereleaved text in "cargo test" output

[dralley]

Problem

The following image is two screenshots, side by side, of the output of "cargo test" from two test runs seconds apart with the same code. As you can see, there seems to be a race (or race-like behavior) causing text written to the stdout buffer to be interleaved together.

This is exclusive to when --nocapture is specified. I cannot reproduce if --nocapture is not specified.

Usually it works "correctly" but I can I can do this over and over and continue getting different versions of incorrectly printed output each time.

Another one

Notes

Output of cargo version:

cargo 1.53.0-nightly (0ed318d18 2021-04-23)
rustc 1.53.0-nightly (42816d6 2021-04-24)

[dralley]

I'm not familiar with the separation of responsibilities, so if this is actually a rustc bug, I will go re-file it over there.

[alexcrichton]

Ah yes this is indeed a rustc issue which defines libtest, not a Cargo issue, I've moved the issue to rust-lang/rust.

[bjorn3]

Isn't this expected behavior? Multiple tests run at the same time on different threads by default. If two threads print something and this printing isn't captured, it is expected to see interleaving. If you don't want interleaving you can use --test-threads 1 to only run a single test at a time.

[dralley]

If that is the case I would say it's not clear from the help text what the implications of --nocapture are, exactly. This was a long while ago so I can't remember exactly what I was doing but I believe I may have been trying to print from directly inside a test or something along those lines. If the only way to do that has other undesirable side effects, this wasn't obvious.

This is what the description says:

   --nocapture     don't capture stdout/stderr of each task, allow printing directly

[epage]

Should --no-capture imply --test-threads 1?

[bjorn3]

As a default, that seems like it could be useful. I don't think it should force single threaded test execution however.